Various types of methods have been developed for introducing an exogenous substance, such as DNA, RNA, a protein, a chemical agent, or the like, into a target cell. Techniques for introducing an exogenous substance into a target cell are broadly divided into biological techniques, chemical techniques, and physical techniques. One known biological technique is a method that uses a virus. Generally known chemical techniques include the calcium phosphate method and the lipofection method. Physical techniques include the electroporation method, the gene gun (particle gun) method, methods that use ultrasound, and the like. It is known that in bacteria such as Escherichia coli and the like, DNA is easily introduced into a cell by applying heat shock to a cell that has been made artificially competent in the presence of calcium chloride. The method of applying heat shock is widely used for the genetic transformation of the E. coli. 
The biological method that uses a virus requires special research facilities for the containment of viral infection. Moreover, the biological method that uses a virus poses the problem that the cell into which the virus is introduced may become cancerous due to the toxicity, antigenicity, and the like of the virus. The calcium phosphate method does not require a special apparatus. The reagents that are necessary in order to apply the calcium phosphate method are inexpensive. However, with the calcium phosphate method, damage to the target cell is comparatively great, and the transfection efficiency is poor. The lipofection method has a better transfection efficiency than does the calcium phosphate method, and it has the advantage of requiring only a small amount of DNA (the exogenous substance). However, with the lipofection method, the number of parameters to be examined is comparatively great, depending on the target cell, and an expensive reagent is required. With the chemical techniques, the toxicity of the reagent limits the types of cells that can be used, and in a case where a cell type such as a hippocampal neuron or the like is used as the target cell, a good transfection efficiency is difficult to achieve.
In comparison to the biological techniques and the chemical techniques that are described above, the advantages of the physical techniques are that toxicity to the cell does not need to be considered and an expensive reagent is not required. With the gene gun method, gold particles that have been coated with an exogenous gene (for example, DNA) are shot into the target cell by high-pressure helium gas, such that the exogenous gene is introduced directly into the nucleus of the target cell. In this manner, the gene gun method is able to introduce an exogenous gene into the target cell and cause the gene to be expressed. That is, the gene gun method is a method that shoots a large number of DNA molecules into the target cell. Therefore, with the gene gun method, the amount of gene expression in a single cell is great, but the expression efficiency is poor in the sense that a large number of cells must be defined as the target cells in order to introduce the gene into a significant number of cells. The methods that use ultrasound are not simple, because they require that conditions be set separately for each type of cell by trial and error.
The electroporation method is the most representative of the physical techniques, and it is a method that causes DNA and the like to be incorporated into the cell by the application of a high-voltage pulse to the cell, such that pores are temporarily created in the cell membrane through which exogenous substances can pass. The electroporation method achieves a higher transfection efficiency than do the chemical techniques, it is easy to perform and is highly safe and reproducible, and it can be applied to various types of organisms (including plant cells) and various types of cells.
The electroporation method is a method in which processing is performed in a state in which electrodes are immersed in a liquid suspension that contains cells and an exogenous substance. Therefore, with the electroporation method, when the processing is performed several times using the same electrodes, there is a strong possibility that a new liquid suspension will be contaminated by the old suspension that has adhered to the electrodes. Contamination can be avoided by discarding the used electrodes after each round of processing, but that increases the cost of the processing.
Furthermore, with the electroporation method, a comparatively large sample volume is required, making it necessary to prepare a large amount of scarce cells. Therefore, with the electroporation method, the possibility exists that analysis will be difficult in a case where the amount of cells within the sample is small. Methods that have been proposed for reducing the amount of the sample that is required include a method for performing electroporation by filling a hollow capillary tube or a tube with a sample (Patent Literature 1), a method for improving the transfection efficiency of the exogenous gene (Patent Literature 2), and a pipette tip type electroporation apparatus (Patent Literature 3). However, in clinical practice, such as in gene therapy, cell transplantation, and the like, it is best to harvest as few cells as possible in order to reduce the burden on the patient, and demand has also grown for further reductions in the amount of high-cost exogenous genes that are used.
In addition, with the electroporation method, an expensive pulse generator is required, and in many cases, studies of the optimum conditions for introducing exogenous substances become complicated.
The inventors have developed an apparatus that is provided with a container that is filled with oil, two electrodes that are affixed in parallel to a bottom face of the container, and a high-voltage direct current power supply, with one of the electrodes serving as a high-voltage electrode and the other electrode serving as a ground electrode, and the apparatus applying voltage from the high-voltage direct current power supply. The inventors discovered that, by placing a hydrophilic liquid droplet that contains plasmid DNA and competent cells of E. coli between the electrodes and applying a voltage, while moving the liquid droplet back and forth between the electrodes, the plasmid DNA (an exogenous substance) can be introduced into the E. coli in the drop of liquid using a sample with a volume of only two microliters. The inventors proposed a new electroporation technology that is able to make an expensive pulse generator unnecessary (Non-Patent Literature 1, 2).